The present invention relates broadly to a safe life monitor apparatus, and in particular to a captive volume device to monitor the safe life of an expendible material.
In the prior art, previous attempts to develop an apparatus such as a safe life monitor for propellant material have been based upon methods which directly sense a change in the propellant material in the propulsion device, i.e. a rocket motor, etc. The problems which exist with the prior art methods and apparatus, is the unreliable methods and means that are utilized to directly measure the small changes in the gassing rates, the mechanical and/or physical properties of expendible material over long periods of time. This type of monitoring and measuring of an expendible material is difficult to do reliably and is affected by the changes in ambient and operating temperature and pressure.
One method of determining and sensing the presence of various gases which is well known and widely accepted involves passing a sample of the gas through an elongate transparent tube in which there is contained a colorimetric indicator. The colorimetric indicator reacts to the presence of the gas sample by changing the color of the colorimeter material in the transparent tube. In order to provide a quantitative measure of the detected gas, the reaction of the colorimeter material to a particular gas sample may be preset to achieve a specific color or develop a particular color stain length. In general, it is required that colorimeter indicator tubes be sealed for storage and, when they are to be used for gas detection and analysis, a portion of the tube will be removed to expose the gas to the colorimeter material.
One of the gases that is useful to detect, is nitrogen oxide. In determining nitrogen oxides, it is common to utilize a granular carrier base with a coating of tetraphenylbenzidine or dimethyldiphenylbenzidine to operate as the indicator. Some prior art indicators have utilized a diphenylbenzidinedecasulfonic acid or a water soluble salt of diphenylbenzidinedecasulfonic acid that is carried by a granular solid. The granular solid carrier which may preferably be a granular absorbent, does not enter into the color-producing reaction. The granular carrier provides an inert physical carrier base for the reagent material. Silica gel is the preferred carrier among the various carriers which are available. However, any water soluble salt of diphenylbenzidine-decasulfonic acid may be used. It may be noted that certain alkali metal salts, such as sodium salts, are generally preferred.
While such indicators as described above are very sensitive to nitrogen oxides in general, they have the distinct disadvantage of very rapidly deteriorating with exposure to oxygen. Thus, these colorimeter materials require very careful handling and any presence of oxygen must be removed from the tubes prior to filling. The preparation of such colorimeter indicators requires great care. For Example, the benzidine component and the granular carrier must be heated to sublime the benzidine component into the carrier and unless this operation is carefully performed, it may result in an uneven distribution of reagent on the support material.
On such useful device or technique which is used as a means of measuring and or monitoring the end of the useful life of materials, such as solid rocket propellants, is the present captive volume device. The captive volume device may comprise a small parasitic unit which is attached to the structure that contains the material whose lifetime is to be monitored. Various types of propellant measuring and or monitoring devices are shown to exist in the prior art.
In the prior art, it may be seen that a colorimetric NO.sub.x detector has been utilized for a propellant, but not in a captive volume device. It may be further seen that colorimetric NO.sub.x detectors are well known and have been used to evaluate mediums other than propellants.
The use of a passive differential pressure sensing device is well known in the prior art. Some such differential pressure sensing devices are the use of snap action diaphragms, reverse buckling diaphragms, rupture disks, reverse buckling rupture disks, belleville springs and bellows. One such prior art device discloses a pressure ratio reversal indicator which includes a sight glass and a reversible diaphragm, although the device is not identified as a captive volume device apparatus. A similar device discloses a fluid pressure indicator which utilizes a diaphragm and sight glass arrangement. The present invention involves a captive volume device to measure the useful life of a solid propellant. This device utilizes a colorimetric NO.sub.x detector contained in a small volume of the captive volume device along with a sample of the propellant. With the nitrate ester stabilizer concentration being adjusted to a lower level than that of the main propellant, the propellant in the captive volume device will reach its gassing point prior to the main batch. The onset of gassing indicates that the end of safe life has been reached in the case of this type of propellant.